1. Field of the Invention
This invention relates to an interferometer, and to a demodulator employing this interferometer.
Priority is claimed on Japanese Patent Application No. 2007-060497, filed on Mar. 9, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
Generally in an interferometer, an incident beam of light is split into a plurality of split beams, and after passing through different optical paths the split beams are caused to interfere, and the interference fringes or similar are measured. One type of interferometer is a delayed interferometer, in which, relative to one split beam, another split beam is delayed, and interference between these beams is caused. Such delayed interferometers are for example provided in the demodulation devices of WDM (Wavelength Division Multiplexing) optical communication systems. A demodulation device in a WDM optical communication system performs wavelength division multiplexing of optical signals modulated by DPSK (Differential Phase Shift Keying) or another modulation method, and transmits the multiplexed signals. Differential phase shift keying is a modulation method in which a signal is modulated by relative phase difference with the phase of the preceding signal to perform modulation.
FIG. 9 and FIG. 10 show the configuration of a delayed interferometer of the conventional art, provided in a demodulation device. The delayed interferometer 100 shown in FIG. 9 is a Mach-Zehnder interferometer, configured with optical lines (optical waveguides). The delayed interferometer 200 shown in FIG. 10 is a bulk-type Mach-Zehnder interferometer, configured with a plurality of optical elements.
The delayed interferometer 100 shown in FIG. 9 includes an input line 101, directional coupler 102, split beam lines 103 and 104, directional coupler 105, and output lines 106 and 107. The input line 101 is an optical line through which differential phase shift keyed WDM light L100 is input. The directional coupler 102 splits the WDM light L100 which has passed through the input line 101 with a prescribed intensity ratio (for example, 1:1).
The split beam lines 103 and 104 are optical lines along which split beams L101 and L102, split by the directional coupler 102, respectively propagate. The optical path lengths of the split beam lines 103 and 104 are set such that the length of the split beam L102 is delayed by a time duration equivalent to one bit of the modulation rate relative to the split beam L101. The directional coupler 105 combines the split beams L101 and L102 via the split beam lines 103 and 104, and causes interference. Further, the directional coupler 105 splits the interference light obtained by interference at a prescribed intensity ratio (for example, 1:1). The output lines 106 and 107 output the interference light split by the directional coupler 105 as output beams L103 and L104.
The delayed interferometer 200 shown in FIG. 10 includes a beam splitter 201, reflecting mirrors 202 and 203, and beam splitter 204. In FIG. 10, light beams equivalent to the WDM light beam L100, split beams L101 and L102, and output beams L103 and L104 in FIG. 9 are indicated using the same symbols. The beam splitter 201 splits the differential phase shift keyed WDM light beam L100 at a prescribed intensity ratio (for example, 1:1). The reflecting mirrors 202 and 203 form a so-called corner cube mirror. The reflecting mirrors 202 and 203 guide one of the split beams L102 to the beam splitter 204 by successively reflecting the split beam L102 split by the beam splitter 201.
The beam splitter 204 combines the other split beam L101 split by the beam splitter 201 with the split beam L102 guided by the reflecting mirrors 202 and 203, to cause interference. Further, the beam splitter 204 splits the inference beam obtained by interference at a prescribed intensity ratio (for example, 1:1). The interference light beams split by the beam splitter 204 are output as output light beams L103 and L104. The beam splitters 201 and 204 and reflecting mirrors 202 and 203 are positioned in relative positions such that the split beam L102 is delayed, relative to the split beam L101, by a time duration equivalent to one bit of the modulation rate of the WDM beam L100.
When WDM light L100 is input to a delayed interferometer 100 or 200 configured as above, the light is split into a split beam L101 and a split beam L102. As a result of propagation of the split beams L101 and L102 over different optical paths, a delay occurs in the split beam L102, by a time duration equivalent to one bit of the modulation rate of the WDM light L100. Then, the split beams L101 and L102 are combined and caused to interfere, and a as result phase comparison is performed between the split beam 101 and the split beam L102 delayed by the time duration described above. The interference light, having intensity according to the comparison result, is output as the output light beams L103 and L104. By this means, demodulation of the different phase shift keyed WDM light L100 (i.e. demodulation of WDM light L100 which is modulated by the relative phase difference with the phase of the preceding signal) is performed.
Details of a demodulation device employing a delayed interferometer of the conventional art in a WDM optical communication system may for example be found in Published Japanese Translation of PCT Application 2004-516743 (PCT Publication No. WO 02/51041).
In order to improve the performance (for example, the optical signal-to-noise ratio (OSNR), Q value, or similar) of the above-described delayed interferometers used in demodulation devices in WDM optical communication systems, the phase of the interferometer must be precisely matched with the carrier light of the transmitting signal. To this end, delayed interferometers are provided with a phase adjustment mechanism for fine adjustment of the phase of the interferometer relative to the carrier light, and the phase adjustment is performed with precision.
Ideally, a splitting element employed by a delayed interferometer (for example, the directional coupler 102 or beam splitter 201) splits an incident light beam at a prescribed intensity ratio, regardless of the polarization state of the incident light (S-polarized or P-polarized), and without causing a relative phase difference between the split beams. However, actual splitting elements deviate from this ideal case. Hence relative phase differences between split beams occur, according to the polarization state of the incident light, arising from imperfections in the splitting element. Due to such phase differences, there is the problem of occurrence of phenomena in which the phase of a delayed interferometer changes depending on the polarization state of the incident light (PDFS: Polarization-Dependent Frequency Shift).
The above polarization-dependent frequency shift cannot be eliminated using the above-described phase adjustment mechanism, and is a factor detracting from the performance of a delayed interferometer. The polarization-dependent frequency shift (PDFS) is not a problem inherent only in delayed interferometers such as the delayed interferometers 100 and 200 shown in FIG. 9 and FIG. 10, but is a problem which occurs in interferometers in general which employ splitting elements such as half-mirrors, beam splitters, and similar.